1. Field of the Invention
The present invention relates to an antenna device equipped with an impedance conversion circuit, and a communication terminal apparatus including the antenna device, and particularly to an antenna device that provides impedance matching in a wide frequency band, and to a communication terminal apparatus including the antenna device.
2. Description of the Related Art
In recent years, communication terminal apparatuses, such as mobile phones, may require compatibility with communication systems, such as a GSM (registered trademark) (Global System for Mobile communications), DCS (Digital Communication System), PCS (Personal Communication Services), and UMTS (Universal Mobile Telecommunications System), as well as a GPS (Global Positioning System), a wireless LAN, Bluetooth (registered trademark), and the like. Thus, antenna devices for such communication terminal apparatuses are required to cover a wide frequency band of 800 MHz to 2.4 GHz.
The antenna devices for a wide frequency band typically have a wideband matching circuit including an LC parallel resonant circuit or an LC series resonant circuit, as disclosed in Japanese Unexamined Patent Application Publication No. 2004-336250 and Japanese Unexamined Patent Application Publication No. 2006-173697. In addition, known examples of the antenna devices for a wide frequency band include tunable antennas as disclosed in Japanese Unexamined Patent Application Publication No. 2000-124728 and Japanese Unexamined Patent Application Publication No. 2008-035065.
However, since each of the matching circuits disclosed in Japanese Unexamined Patent Application Publication No. 2004-336250 and Japanese Unexamined Patent Application Publication No. 2006-173697 includes a plurality of resonant circuits, the insertion loss in the matching circuit is likely to increase and a sufficient gain may not be obtained in some cases.
On the other hand, since the tunable antennas disclosed in Japanese Unexamined Patent Application Publication No. 2000-124728 and Japanese Unexamined Patent Application Publication No. 2008-035065 require a circuit for controlling a variable capacitance element, that is, a switching circuit for switching frequency bands, the circuit configuration is likely to be complicated. Besides, since loss and distortion in the switching circuit are large, a sufficient gain may not possibly be obtained in some cases.